Apparatus for shaping a workpiece by electrical discharge with detection means for stopping the shaping operation under abnormal conditions

ABSTRACT

An apparatus for shaping a workpiece by electrical discharge in a gap between the workpiece and a shaping electrode. An oil pressure servo-mechanism is used to maintain the gap at a constant value. The shaping process takes place within a tank filled with an inflammable liquid. Under abnormal conditions, an undesirable deposit, such as carbon, may form and build up on the workpiece surface and thereby cause the electrode to move away from the workpiece during a shaping process instead of closer thereto. Means, such as a mechanical switching arrangement or electrical circuitry, are provided for detecting time delay the abnormal condition and for enabling the shaping operation to thereby be stopped.

United States Patent Saito et al.

APPARATUS FOR SHAPING A WORKPIECE BY ELECTRICAL DISCHARGE WITH DETECTIONMEANS FOR STOPPING THE SHAPING OPERATION UNDER ABNORMAL CONDITIONS NagaoSaito; YoichiKuji; Sinii Arai, all of Nagoya, Japan Mitsubishi ElectricCorporation, Tokyo, Japan Filed: Aug. 14, 1970 Appl. No.: 63,760

Inventors:

Assignee:

U.S. Cl. ..2l9/69 S, 219/69 G Int. Cl. ..B23p 1/08, B23p 1/14 Field ofSearch ..219/69 C, 69 G, 69 P, 69 S [4 1 Apr.4, 1972 [56] ReferencesCited UNITED STATES PATENTS 3,439,146 4/1969 Ullmann et al ..2l9/69 GPrimary Examiner-R. F. Staubly Attorney0blon, Fisher & Spivak [57]ABSTRACT An apparatus for shaping a workpiece by electrical discharge ina gap between the workpiece and a shaping electrode. An oil pressureservo-mechanism is used to maintain the gap at a constant value. Theshaping process takes place within a tank filled with an inflammableliquid. Under abnormal conditions, an undesirable deposit, such ascarbon, may form and build up on the workpiece surface and thereby causethe electrode to move away from the workpiece during a shaping processinstead of closer thereto. Means, such as a mechanical switchingarrangement or electrical circuitry, are provided for detecting timedelay the abnormal condition and for enabling the shaping operation tothereby be stopped.

7 Claims, 7 Drawing Figures PATENTEBAPR 41912 I V 3, 54,420

' sumeurs NAGAO SAITO YOICHI KUJI INVENTORS g SINJ! ARAI 1g? v 051Fir/mum ATTORNEYS OPERATIONAL AMPLIFIERS 27, 28, 29

PATENTEIJIPR 41972 I 3,654,420

' sum 3 or 3 0 (QUANTITY OF FLOW OF CYLINDER OIL) FIG 5IISERVO-CQILCURRENTI DOWN I ABN R P UPPER D|RECT|ON (OISERVO-CQILCURRENTo ROCESSING DOWN DIRECTION m mm Z/J W/ W/TIME(b)SERVO-COILCURRENTINTEGRATED VALUE (SW TIME- CHARACTERISTICS OF OUTPUTVOLTAGE INPUT VOLTAGE 0 INVEIQ'I 'ORS NAGAO SAITO I YOICHI KUJI FIG 7SINJIARAI PHASE INVERSION av 05m, HM. Sag/m j ATTORNEYS APPARATUS FORSHAPING A WORKPIECE BY ELECTRICAL DISCHARGE WITH DETECTION MEANS FORSTOPPING THE SHAPING OPERATION UNDER ABNORMAL CONDITIONS BACKGROUND OFTHE INVENTION Field Of The Invention This invention generally relates toan apparatus for shaping a workpiece by electrical discharge inaccordance with the shape of an electrode and more particularly to anapparatus for shaping a workpiece by providing an arc discharge withinthe gap between the workpiece and the electrode which apparatus ischaracterized with the provision for automatically detecting an abnormalshaping condition, such, for example, as a condition wherein theelectrode will move in a direction opposite to that of the shapingdirection as a result of a deposit, such as carbon, being formed withinthe shaping gap.

Description Of The Prior Art FIG. 1 shows the relative relationshipwhich exists between an electrode 3 and a workpiece 2 in a typicalapparatus for shaping by electrical discharge. A servo-mechanism (notshown) is generally employed for maintaining the working gap 4 betweenthe electrode 3 and the workpiece 2 at a constant value. Thus, duringshaping of the workpiece 2, the servomechanism will maintain the size ofthe gap 4 at a constant value as the electrode 3 moves in a direction,as shown by the arrow, towards the workpiece.

One of the problems which exists during the shaping of a workpiece isthat sometimes the shaping liquid 5 which passes within the electricaldischarge gap 5 either is unstable, has insufficient insulationrecovery, or a powder is formed and retained within the gap such as tocause the electrical discharge to focus at a point on the electrodesurface and result in a stationary arc condition. Under such adverseconditions, if, for example, kerosene is used as the shaping liquid, thesame may become decomposed by the electrical discharge and a separatecarbon buildup may be accumulated within the shaping gap 4. Thiscondition is more clearly shown with reference to FIG. 2.

In FIG. 2, the accumulated deposit, such as carbon, is shown as G. Intypical apparatus for shaping a workpiece by electrical discharge, thesize of the gap 4 between the electrode 3 and the workpiece 2 isdetected by measuring the average electrical discharge shaping voltage.Thus, under normal shaping conditions, the detected shaping voltage willenable a servo-mechanism to adjust the gap 4 between the electrode 3 andthe surface of the workpiece 2 by moving the electrode 3 in a directiontowards the workpiece 2 in accordance with the amount of surface removedfrom the workpiece.

Now, under the adverse conditions enumerated above, a deposit G may formand thereby cause the electrical discharge to occur between the depositG and the electrode 3, instead of between the workpiece 2 and theelectrode 3. As such, the average electrical discharge shaping voltagewhich is detected as corresponding to the gap will erroneously be thevoltage between the electrode 3 and the deposit G. As a direct resultthereof, the servo-mechanism will cause the electrode 3 to move inaccordance with the erroneously detected shaping voltage in a directionaway from the workpiece 2, the amount being dependent upon the size ofthe deposit G. If such a condition continues for a long period of time,the point of electrical discharge will come near the surface of theshaping liquid 5 and a fire may result therefrom. Obviously, thepossibility of a fire is quite undesirable, both from a standpoint ofsafety and from a standpoint of shaping efficiency.

SUMMARY OF THE INVENTION Accordingly, it is one object of this inventionto provide a new and improved apparatus for detecting an abnormalcondition during the shaping of a workpiece by electrical discharge.

A further object of this invention is to provide a new and improvedunique apparatus for shaping a workpiece by electrical discharge betweenthe workpiece and a shaping electrode which includes mechanicalswitching means for detecting an abnormal condition and for stopping theshaping process at such time.

Yet another object of the present invention is to provide a new andimproved unique apparatus for shaping a workpiece by electricaldischarge which includes means for detecting an abnormal conditionthrough the use of electrical circuitry and for stopping the shapingprocess at such time.

One other object of this invention is the provision of a new andimproved unique apparatus for enabling the operation of shaping aworkpiece to be stopped with a high degree of reliability upon thedetection of an abnormal condition whereby the possibility of fires maybe eliminated and the shaping efficiency increased.

Briefly, in accordance with this invention, these and other objects arein one aspect attained in an electrical discharge shaping apparatuswherein the shaping occurs within a liquid by the provision of means fordetecting movement of a given amount of the shaping electrode in adirection opposite to that of the shaping direction and for stopping theshaping operation if the detection continues after a predetermined timedelay.

BRIEF DESCRIPTION OF THE DRAWINGS A more complete appreciation of theinvention will become readily apparent as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 shows a sectional view of a typical relationship between ashaping electrode and a workpiece in a normal electrical dischargeshaping apparatus;

FIG. 2 shows a sectional view of a typical relationship between ashaping electrode and a workpiece under abnormal conditions, such as fordeposit build-ups within the working gap, in an electrical dischargeshaping apparatus;

FIG. 3 shows a schematic block diagram of one preferred embodiment ofthe electrical discharge shaping apparatus in accordance with thepresent invention;

FIG. 4 shows a schematic block diagram of an alternative preferredembodiment of the electrical discharge shaping apparatus according tothe present invention;

FIG. 5 is a characteristic curve which shows the quantity of flow ofcylinder oil versus servo-coil current for the oil pressureservo-mechanism of the present invention;

FIG. 6(a) is a characteristic curve showing the servo-coil currentplotted against time;

FIG. 6(b) is a characteristic curve showing the integrated value of theservo-coil current plotted against time; and,

FIG. 7 is a characteristic curve showing the output voltage plottedagainst the input voltage of an operational amplifier used in theembodiment shown in FIG. 4 of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to thedrawings, wherein like reference numerals designate identical, orcorresponding parts throughout the several views, and more particularlyto FIG. 3 thereof, wherein one embodiment of the present invention isshown as including a shaping tank 1, a workpiece 2 which is fixed withinthe tank 1 and a shaping electrode 3 for shaping the workpiece 2 througha working gap 4. A shaping liquid 5, such, for example, as kerosene, isprovided and the workpiece 2 and shaping electrode 3 are immersedtherein. The electrode 3 is attached to one end of an elongated rod 6and the other end of the rod 6 is connected to a piston of an oilpressure servo 7, which is provided for moving the electrode 3. Theservo 7 also includes an oil pressure cylinder 8 and a valve 10, such asof the electromagnetic type which includes a coil 11. A potentiometer 12having a reference power source 13 connected thereacross is connectedthrough the coil 11 between the workpiece 2 and the shaping electrode 3.In the event that the gap voltage becomes greater than the referencevoltage supplied through the potentiometer, the coil 11 will beenergized and cause the electrode 3 to move towards the workpiece 2 byincreasing the oil pressure of the cylinder 8, in the direction as shownby the solid arrows, and thereby move the elongated rod 6 towards theworkpiece 2 until the desired constant gap size is reached. in a similarmanner, the rod 6 will move the electrode 3 away from the workpiece 2 inthe event that the shaping voltage falls below the reference voltage byincreasing the oil pressure in a direction as shown by the dottedarrows.

The apparatus also includes an auxiliary elongated rod 15 which isparallel to the electrode moving rod 6. A movable contact plate 16 isconnected to the auxiliary rod 15 and will move when subjected to a loadgreater than that of a given value. A limit switch LS, such as of themicro type, is attached to the contact plate 16 and is normally in anopen condition (hereinafter referred to as open contact LSa). A contactmaking stem 17 is supported upon the movable rod 6 and forms a part ofthe limit switch LS.

The apparatus of the present embodiment further includes an AC powersource 20 and a pulse generator 21 which pushbutton specific polaritysquare wave pulses. The positive output terminal of the generator 21 isconnected to the electrode 3 and the negative output terminal thereof isconnected to the workpiece 2. An electromagnetic contactor MC havingnormally open contacts MCal and MCa2, a time limit relay RT having anormally closed contact RTb for providing a delay operation andpush-button switches Pb l and Pb 2 are additionally provided.push-button A detailed description of the operation of the apparatus ofthis embodiment of the invention will now follow. When the pushbuttonswitch Pbl is closed, the electromagnetic contactor MC will cause thenormally open contacts MCal and MCa2 to close. The contact MCa2 servesas a self-maintenance circuit for operating the electromagneticcontactor MC When the contact MCal is in a closed position, the ACelectric power source 20 will supply power to the pulse generator 21 sothat square wave pulses will be obtained at the output terminalsthereof. The square wave pulses are then supplied between the workinggap 4 to result in an electrical discharge therein. The power supply tothe pulse generator 21 may be turned off by opening the push-buttonswitch Pb2 or the contact RTb which will, in turn, open the contactsMCal.

Under a normal shaping operation, such as shown in FIG. 1, the electrode3 will automatically move in a direction towards the workpiece 2 as thesame is shaped. The desired movement of the electrode 3 will be providedby the oil pressure servomechanism 7. Under such normal conditions, thecontact plate 16, shown in FIG. 3, will move in a downward directionwith the contact closure stem 17 remaining in contact with the loweredge of the contact plate. However, in the event that adverse conditionsoccur and a deposit G, such as carbon, is accumulated within the shapinggap 4, as shown in FIG. 2, then the electrical discharge will occurbetween the deposit G and the electrode 3 and, in turn, theservo-mechanism 7 will operate to erroneously maintain constant the gapbetween the deposit G and the electrode 3, instead of the gap betweenthe electrode 3 and the workpiece 2, so as to cause the electrode 3 tomove in an upward direction as earlier described. As the electrode 3 ismoved in an upward direction, the stem 17 will eventually make contactwith the upper edge of the contact plate 16 and thereby close the limitswitch LS and thus normally open contact LSa. The time limit relay RTwill then begin to operate and if the limit switch LS remains closed formore than the set delay time of the relay RT, then the normally closedcontact RTb of the relay RT will open and turn off the operation of theelectromagnetic contactor MC. With the contactor MC turned off, thecontacts MCal and MCa2 will be opened and the electric power beingsupplied to the pulse generator 21 will be turned off with the resultthat the electrical discharge within the gap will be stopped. It shouldbe understood that the purpose of the delay in turning off the power isto allow for the possibility of the limit switch LS briefly being closedwhen transferring from a downward to an upward movement caused by achange in direction of the servo-mechanism 7.

It should now be apparent that with the present invention, the apparatuswill shut down before a deposit G is accumulated, as shown in FIG. 2, tosuch an extent that an electrical discharge could occur near the surfaceof the shaping liquid and result in sparks in the air which could causea fire to occur. Ordinarily, the electrical discharge shaping can safelybe operated with a flammable liquid in that the electrical dischargewill occur far away from the surface thereof and in the absence of air.

The embodiment shown and described with reference to FIG. 3 enables thedetecting of an upward movement of the electrode 3 by the use of amechanical limit switch. Obviously, the invention is not so limited.Thus, in FIG. 4, an alternative and preferred embodiment of the presentinvention shows circuitry for electrically detecting an upward movementof the electrode 3. Such circuitry by eliminating movement furtherincreases the reliability of the apparatus.

Referring now to FIG. 4, the electrical discharge shaping apparatus isshown as including similar to the embodiment of FIG. 3 the shaping tank1, the workpiece 2 which is fixed within the tank 1, and the shapingelectrode 3 for shaping the workpiece 2 through the working gap 4. Theshaping liquid 5, such, for example, as kerosene, is provided and theworkpiece 2 and electrode 3 are immersed therein. The electrode 3 isattached to one end of the elongatedrod 6 and the other end of the rod 6is connected to the piston of the oil pressure servo 7 which is providedfor moving the electrode 3. The servo 7 also includes the oil pressurecylinder 8 and the valve 10, such as of the electromagnetic type whichincludes the coil 11. The potentiometer 12 having a reference powersource 13 connected thereacross is connected through the coil 11 betweenthe workpiece 2 and the shaping electrode 3. In the event that the gapvoltage becomes greater than the reference voltage supplied through thepotentiometer, the coil 11 will be energized in a direction shown by thearrow 22 and cause the electrode 3 to move towards the workpiece 2 byincreasing the oil pressure of the cylinder 8 as shown by the solidarrow 23 and thereby move the rod 6 towards the workpiece 2 until thedesired constant gap size is reached. In a similar manner, the elongatedrod 6 will move the electrode 3 away from the workpiece in the eventthat the shaping voltage falls below the reference voltage by increasingthe oil pressure in a direction as shown by the dotted arrows.

The apparatus of the embodiment of FIG. 4 also includes the three-phaseAC power source 20 for supplying electric power to the pulse generator21 upon closure of the normally open contact MCal of the electromagneticcontactor MC. The pulse generator 21 will supply square wave pulses atthe positive terminal 24 thereof to the electrode 3 and will supplypulses to the workpiece 2 over the negative terminal 25.

The operation of the apparatus described above for shaping a workpieceby electrical discharge will now be described. When the push-buttonswitch Pbl is placed in the on position, the coil of the electromagneticcontactor MC will be excited and the normally opened contact MCal willbe closed. The closing of the contacts MCal will enable the electricpower from the source 20 to be supplied to the pulse generator 21 andsimultaneously contact MCa2 will be closed and constitute the selfmaintenance circuit for the magnetic contactor MC after the switch Pb1is opened. The apparatus of the present embodiment also includes theclosed contact pushbutton switch Pb2 and the contact RTb of the closedcontact time limit relay RT. A relay RA with an open contact RAa isprovided and the same will'allow the relay contact RTb to open after aspecific time delay. The opening after the time delay of the relaycontact RTb will stop pulses from being supplied to the working gap fromthe pulse generator 21. The stopping of pulses from being applied to theworking gap may also be effected by placing the push-button switch Pb2in the on position.

In the present embodiment, a resistor 26 is provided for enabling avoltage to be developed thereacross in an amount proportional to theelectrical current flowing in the servo-coil 11. The voltage e acrossthe resistor 26 is provided by the following formula:

2 iRo 1 1 wherein iA is the electric current in the servo-coil 11 andRois the resistance of the resistor 26. The polarity of the voltage eacross the resistor 26 will change in accordance with the direction ofthe electrode 3 and the magnitude thereof will change in accordance withthe speed of movement of the electrode 3. When the electrode 3approaches in a direction towards the workpiece 2, the electricalpotential at the right hand side terminal point of the resistor 26 willbe of a high value, while on the other hand, when the electrode 3 movesin a direction so as to recede from the workpiece 2, the electricalpotential on the left hand terminal side of the resistor 26 will be of ahigh value.

Referring now to FIG. 5, the relationship between the amount of currentpassing through the servo-coil 11 to the quantity and direction of fiowof oil passing to the cylinder portion 8 of the servo-mechanism 7 istherein graphically shown. In FIG. 5, the quadrant designated upcorresponds to the case wherein the electrode 3 is receding from theworkpiece 2 and the quadrant designated down refers to the case whereinthe electrode 3 moves in a direction so as to approach the workpiece 2.

F [6. 6a graphically shows the servo-coil current versus time undervarious conditions during a shaping operation. For example, in a normalshaping operation, the average servo-coil current will be in a downwarddirection, even though at times the electrode 3 will briefly move in anupward direction. Under such normal shaping conditions, the electrode 3will move in a direction towards the workpiece 2. However, when abnormalconditions are present and a deposit, such as carbon, accumulates withinthe working gap, the electrode will move in an upward direction awayfrom the workpiece 2 and the servo-coil current will flow only in theupward direction as shown in that portion of FIG. 6a designated abnormalprocessing.

Referring now to FIG. 6b, the integrated value of the servocoil currentof FIG. 6a is shown as being plotted against time. From the Figure, itshould be readily apparent that during a normal shaping operation, e.g.,when the electrode is moving towards the workpiece 2, the integratedservo-coil current will be of a negative value as time proceeds, whileduring an abnormal shaping operation. the integrated value willeventually turn positive. The apparatus of the present inventionutilizes the integrated value ofthe servo-coil current to detect amovement of the electrode 3 in a direction away from the workpiece 2 toallow the apparatus to be stopped at an appropriate time.

Referring again to H0. 4, it should be again noted that the voltage edeveloped across the resistor 26 is of an amount proportional to theservo-coil current. In FIG. 4, a plurality of conventional operationalamplifiers 27, 28, and 29 are provided and the same have the phaseinversion transfer characteristics as shown in FIG. 7. The voltage e isapplied to the operational amplifier 27, through a resistor R and theoutput thereof which is designated as e may be represented by thefollowing formula:

The amount of amplification of the operational amplifier 27 isdetermined by the ratio of the feedback resistor R to the resistor RWhen abnormal conditions exist and the electrode 3 is moved in an upwarddirection away from the workpiece 2, the servo-coil current will flow ina direction as indicated by the dotted arrow 22 and as such the outputvoltage from the amplifier 27 will be of a negative value. In order toequalize the phase at the output of the amplifier 27 for enabling asignal suitable for later use, operational amplifier 28 for the purposesof inversion is supplied. The output voltage of the operationalamplifier 28 is designated by e and may be represented by the followingformula:

The integrated value of the voltage e across the resistor 26 willchange, as shown in FIG. 6b, and as seen by the formula (4) will be of apositive value during an abnormal shaping operation, namely, when theelectrode 3 is receding from the workpiece 2. A Zener diode 30 isprovided and will normally be in a non-conductive state, such, forexample, as when the output voltage of the integrating circuit 29 is ofa low or negative value. However, when he voltage e at the output of theintegrating circuit 29 which is given by the formula (4) becomes of apositive value, then the Zener voltage will eventually be reached andthe diode 30 will conduct. Upon conduction of the diode 30, the voltagethereacross will be dropped between a resistor R and a resistor R, whichis connected to the base of a normally non-conducting transistor 31. Thetransistor 31 which is normally non-conductive will conduct upon theapplication of a base current thereto through resistor R upon conductionof the Zener diode 30. Upon conduction of the transistor 31, a collectorcurrent will be provided therefrom and the same will serve to excite therelay coil RA. A diode 32 is provided across the relay RA and serves toprevent the same from being actuated during the instant of time when thetransistor 31 is changed from a conductive to a non-conductive state.The points is designated +E and E are terminals to which a DC powersource may be applied for operating the operational amplifiers 27, 28,and 29 and the transistor 31.

It should be understood that the integrating circuit 29 has enough delayto enable an abnormal upward transfer of the electrode 3 to be detected.However, in order to prevent the apparatus from responding to a normalupward transfer of the electrode 3, an additional delay is supplied tothe apparatus through the use ofa delay time limit relay RT which isexcited after the relay coil RA. Upon excitation of the relay coil RA,the open contact RAa will close and the closed contact RTb will open,after the predetermined set delay. The contact MCal will, in turn, open,and the electric power being supplied to the pulse generator 21 will beturned off so that the operation of shaping by electrical dischargewithin the working gap area will be stopped.

It should be understood that during the period of delay of the delaytime limit relay RT that the contact RAa should be closed. If, at anytime, the contact RAa is opened, the delay operation of the relay RTwill be stopped, and the period of delay before the shaping operation isstopped will have to begin again after the contact RAa is again closed.Accordingly, even though the electrode 3 moves in an upward position fora period of time such that the value integrated at the output of theintegrating operational amplifier 29 is over the value needed to causethe Zener diode to conduct that if the upward transfer of the electrode3 is stopped within a period of time less than that of the delay time ofthe relay RT that the downward movement of the electrode 3 will causethe contact RM to open and allow for shaping operation to continue.Thus, with the present embodiment, the shaping operation will only bestopped if the electrode 3 moves in the upward condition for a givenperiod of time such that an abnormal condition will clearly be detected.

Thus, with the present embodiment the provision of double delays by theintegrating circuit 29 and the time limit relay circuit RA enables theapparatus to respond to only a continuous abnormal upward transfer ofthe electrode 3 which could be caused by a deposit accumulation, such ascarbon, but will not allow the apparatus to respond to an upwardtransfer of the electrode 3 which is merely caused by a sudden shortbetween the electrode 3 and the workpiece 2 or an intermittent upwardtransfer of the electrode 3 which may be caused during the supplying ofthe shaping liquid within the working gap.

It should now be apparent that with the present embodiment fires whichmay be caused by an electrical discharge near the surface of a shapingliquid may be readily avoided by allowing the apparatus to automaticallybe stopped upon the detection of an abnormal accumulation of a depositof carbon or the like within the shaping area.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. For example, the timeconstant of the integrating circuit 29 can be adjusted so as to providea delay of about 1,000 seconds and by such provisions, the time limitrelay can be eliminated. It is therefore to be understood that withinthe scope of the teachings herein and the appended claims, the inventionmay be practiced otherwise than as specifically described.

What is claimed and desired to be secured by letters patent of theUnited States is:

1. An apparatus for shaping a workpiece immersed within a liquidcontaining tank by electrical discharge comprising:

a shaping electrode placed within said tank and positioned opposite tosaid workpiece so as to provide a gap between said electrode and saidworkpiece;

means for supplying a source of electrical power to said gap during theshaping of said workpiece;

means for maintaining said gap at a constant value as the shapingoperation proceeds; and,

means for detecting when an abnormal condition exists during saidshaping operation and for stopping the same, said means for detectingand stopping including switching means operative in response to amovement of said electrode of a given distance for a predeterminedperiod of time in a direction opposite to the normal shaping directionof said electrode.

2. An apparatus for shaping a workpiece as in claim 1, wherein saidswitching means is of the micro-switch type and includes a contactclosure stem coupled to said electrode and movable therewith such thatsaid stem will make contact with the contact closure portion of anadjoining plate of said switching means upon said movement of saidelectrode of a given distance in a direction opposite to the normalshaping direction of said electrode.

3. An apparatus for shaping a workpiece as in claim 2, wherein saidpredetermined period of time is determined by a delay relay which isprovided and actuated upon operation of said switching means wherebysaid relay will delay the stopping of said shaping operation for apredetermined period of time after which time said operation will bestopped by preventing said source of electrical power from being appliedto said gap.

4. An apparatus for shaping a workpiece immersed within a liquidcontaining tank by electrical discharge comprising:

a shaping electrode placed within said tank and positioned opposite tosaid workpiece so as to provide a gap between said electrode and saidworkpiece;

means for supplying a source of electrical power to said gap during theshaping of said workpiece;

means for maintaining said gap at a constant value as the shapingoperation proceeds, said means for maintaining said gap at a constantvalue being an oil pressure servomechanism; and,

means for detecting when an abnormal condition exists during saidshaping operation and for stopping the same, said means for detectingand stopping including means for integrating the value of time,fluctuations of current in I a va ve coil of said servo-mechanism andwherein upon said integrated value being of a given polarity in valuefor a predetermined time, said abnormal condition will be indicated andthe source of electrical power being supplied to said gap will be turnedoff.

5. An apparatus for shaping a workpiece as in claim 4, wherein saidmeans for integrating includes an operational amplifier.

6. An apparatus for shaping a workpiece as in claim 5, wherein theoutput of said operational amplifier is connected to a semiconductiveswitch which enables a relay to be energized upon conduction of saidswitch, said conduction being caused by movement of said electrode in adirection opposite to that of said shaping operation and wherein if saidupward movement of said electrode continues for a predetermined delaytime provided by said relay, an abnormal condition will be indicated andsaid shaping operation stopped.

7. An apparatus for shaping a workpiece as in claim 6, wherein saidsemiconductive switch is a Zener diode.

1. An apparatus for shaping a workpiece immersed within a liquidcontaining tank by electrical discharge comprising: a shaping electrodeplaced within said tank and positioned opposite to said workpiece so asto provide a gap between said electrode and said workpiece; means forsupplying a source of electrical power to said gap during the shaping ofsaid workpiece; means for maintaining said gap at a constant value asthe shaping operation proceeds; and, means for detecting when anabnormal condition exists during said shaping operation and for stoppingthe sAme, said means for detecting and stopping including switchingmeans operative in response to a movement of said electrode of a givendistance for a predetermined period of time in a direction opposite tothe normal shaping direction of said electrode.
 2. An apparatus forshaping a workpiece as in claim 1, wherein said switching means is ofthe micro-switch type and includes a contact closure stem coupled tosaid electrode and movable therewith such that said stem will makecontact with the contact closure portion of an adjoining plate of saidswitching means upon said movement of said electrode of a given distancein a direction opposite to the normal shaping direction of saidelectrode.
 3. An apparatus for shaping a workpiece as in claim 2,wherein said predetermined period of time is determined by a delay relaywhich is provided and actuated upon operation of said switching meanswhereby said relay will delay the stopping of said shaping operation fora predetermined period of time after which time said operation will bestopped by preventing said source of electrical power from being appliedto said gap.
 4. An apparatus for shaping a workpiece immersed within aliquid containing tank by electrical discharge comprising: a shapingelectrode placed within said tank and positioned opposite to saidworkpiece so as to provide a gap between said electrode and saidworkpiece; means for supplying a source of electrical power to said gapduring the shaping of said workpiece; means for maintaining said gap ata constant value as the shaping operation proceeds, said means formaintaining said gap at a constant value being an oil pressureservo-mechanism; and, means for detecting when an abnormal conditionexists during said shaping operation and for stopping the same, saidmeans for detecting and stopping including means for integrating thevalue of time fluctuations of current in a valve coil of saidservo-mechanism and wherein upon said integrated value being of a givenpolarity in value for a predetermined time, said abnormal condition willbe indicated and the source of electrical power being supplied to saidgap will be turned off.
 5. An apparatus for shaping a workpiece as inclaim 4, wherein said means for integrating includes an operationalamplifier.
 6. An apparatus for shaping a workpiece as in claim 5,wherein the output of said operational amplifier is connected to asemiconductive switch which enables a relay to be energized uponconduction of said switch, said conduction being caused by movement ofsaid electrode in a direction opposite to that of said shaping operationand wherein if said upward movement of said electrode continues for apredetermined delay time provided by said relay, an abnormal conditionwill be indicated and said shaping operation stopped.
 7. An apparatusfor shaping a workpiece as in claim 6, wherein said semiconductiveswitch is a Zener diode.